There are three ways you can
calculate rocket altitude ... performance, prediction
and recording.

Performance requires a device to measure the angle the rocket
reaches from a known distance from the launch pad (most accurate if you
can keep the angle below 45 degrees) and a mathematical formula.

Making
the calculations will require a calculator that has trigonometric
functions (sin, cos, tan). The formula is

(Tangent
of angle) x (Distance from launch pad) = Rocket Altitude

Example 1:
If you are one hundred yards (300 feet) away from the pad, and you
measure an angle of 45 degrees, the tangent of 45 = 1

1 x 300 = 300 feet altitude

Example 2:
If you are five hundred yards away from the pad, and you measure an
angle of 39 degrees, the tangent of 39 = 0.810

0.810 x 500 yards = 405 yards =
1,215 feet altitude

Example 3:
This will illustrate the difference with errors above 45 degrees.
If you are five hundred feet away from the pad, and you measure an angle
between 73 and 74 degrees, the tangent of 73 = 3.27, and the tangent of
74 = 3.48

3.27
x 500 feet = 1,635 feet altitude

3.48
x 500 feet = 1,740 feet altitude

Here you can see a difference of 105 feet in just one
degree of difference. Since the tangent of 90 degrees is indefinable
(technically it's infinity), a one degree difference between 88 and 89
at the same 500 foot distance from the launch pad shows a 14,327 foot
difference. The numerical representation for tangent angles
greater than 45 degrees goes up exponentially, which is why you get the
most accuracy at angles less than 45 degrees. Try and move at
least as many feet away as you would expect the rocket to fly (of
course, if you expect it to go up to 1,000 feet, this may be
impractical).

The Prediction method is an inexpensive (free) way to predict a
rockets expected altitude using a computer program. I use the altitude prediction program ALTPRED.
It is a zipped freeware Windows based program that
has all the Estes motors built into it's database, and the zipped file
includes the APD files for you to import high-power data.All you need to do it input the diameter of the rocket, it's
empty weight, and select a motor and it will give you all the details of
the flight ... altitude, speed, etc.Granted this is only a prediction program, and there are many
variables that can affect the actual performance of the rocket but,
personally, I don't need complete accuracy ... I don't work for NASA,
and I can't tell the difference between a rocket that hits 1,200 feet
versus one that hits 1,300 feet.

The Recording method is the most accurate method, because you
utilize on-board electronics to calculate the actual altitude that the
rocket reaches. Of course, adding electronics to your rockets adds
a hefty expense and makes the design and construction more complicated,
but the rewards are well worth it. In addition to having the
maximum altitude of the rocket recorded, there is also included
electronics to automatically deploy your parachutes at pre-determined
altitudes. There are several excellent recording/chute deployment
altimeters available through ADEPT
Rocketry. I use these electronics in conjunction with the ALTPRED
program to verify the expected altitude. Using actual recording
data, I can tweak the prediction program to give me a more accurate
prediction.